In an elevator installation, an elevator car and a counterweight are conventionally supported on and interconnected by traction means. The traction means is driven through engagement with a motor-driven traction sheave to move the car and counterweight in opposing directions along the elevator hoistway. The drive unit, consisting of the motor, an associated brake and the traction sheave, is normally located in the upper end of the elevator hoistway or alternatively in a machine room directly above the hoistway.
Safety of the elevator is monitored and governed by means of a safety circuit or chain containing numerous contacts or sensors. Such a system is disclosed in U.S. Pat. No. 6,446,760. Should one of the safety contacts open or one of the safety sensors indicate an unsafe condition during normal operation of the elevator, a safety relay within the safety circuit transmits a signal to an elevator control which instructs the drive to perform an emergency stop by immediately de-energizing the motor and applying the brake. The elevator usually cannot be called back into normal operation until the reason for the break in the safety circuit has been investigated and the relevant safety contact/sensor reset. A similar circuit is described in EP-A1-1864935 but instead of signaling an emergency stop through the control, a drive relay and a brake relay are connected in series to the safety chain so that if one of the safety contacts opens the drive relay and brake relay immediately open to de-energize the drive and release the brake, respectively.
Traditionally, steel cables have been used as traction means. More recently, synthetic cables and belt-like traction means comprising steel or aramid cords of relatively small diameter coated in a synthetic material have been developed. An important aspect of these synthetic traction means is the significant increase in the coefficient of friction they exhibit through engagement with the traction sheave as compared to the traditional steel cables. Due to this increase in relative coefficient of friction, when the brake is applied in an emergency stop for an elevator employing synthetic traction means there is a significant increase in the deceleration of the car, which severely degrades passenger comfort and could even result in injury to passengers.